In the realm of computing, the relationship that drives most useful applications is the client-server relationship. The interaction between client and server allows most computing beyond an unconnected, single computer. The client-server relationship defines an architecture in which a user's computer, which may be a personal computer (PC), may be the client machine or entity requesting something from a server, which is the supplying machine or entity. However, a PC may also operate as the server side of the client-server relationship. Both are typically connected via some kind of network, such as a local area network (LAN) or wide area network (WAN).
In the client-server model, the client typically processes the user interface (WINDOWS™, MACINTOSH™, etc.) and may perform some or all of the application processing. Servers may range in capacity from high-end PCs to mainframes. A database server typically maintains databases and processes requests from the client to extract data from or to update the database. An application server, which is a software server, typically provides additional business processing for the clients.
While many client-server models are now commonly referred to as “Web based” and/or “Web enabled,” the architecture is conceptually the same. Users' PCs may still be clients, and there are tens of thousands of Web servers throughout the Internet delivering Web pages and other functionality. On the Web, the client typically runs the browser and, just like legacy client/server systems, can perform a little or a lot of processing, such as simply displaying hypertext mark-up language (HTML) pages, processing embedded scripts, or considerable processing with JAVA™ applets. A myriad of browser plug-ins provide all sorts of possibilities for client processing.
The server side of the Web is typically a multi-tiered server architecture with interlinked Web servers, application servers, database servers, and caching servers. In developing network applications that are offered on the Web, the developer typically codes all aspects of communication between the client and server. Actions intended for the client may depend on responses or actions on the server. Similarly actions on the server may depend on actions or responses from the client. This architecture produces an asynchronous event model. If step 1 is dependent on a response from the server, the process grinds to a halt, and step 2 will be delayed until the response for step 1 has been received.
The programming model for dealing with the asynchronous nature of the client-server architecture may be awkward even to experienced programmers. Therefore, the development of applications which include client-server interaction have generally been reserved for experienced programmers.